Chen Weijie

The introduction of both PMAI and 2PACz into the precursor effectively cleaves edge-shared Pb-I octahedra to facilitate the transformation from PbI₂ to PbI₃ ⁻ complexes as prenucleation clusters, and thus produces much larger colloidal particles with accelerated nucleation. Simultaneously, the crystallization in both spin-coating and annealing processes is significantly postponed owing to the stronger interaction between perovskite and binary additives with more functional groups. Benefiting from such rapid nucleation and slow crystallization, high-quality perovskite layer with larger-sized crystals and fewer defects is formed, and the corresponding device could achieve a high efficiency of 26.05% (certified 25.49%).

Abstract

Fabricating high-quality perovskite layers is essential for achieving high-performance solar cells. Considering the significant advancements made in additive engineering for optimizing perovskite crystallization using single additive, exploring the collaborative effect of dual additives on precursor for perovskite crystallization may be an effective way for further advancing device performance. Herein, a binary additives strategy is proposed, where phenylmethylammonium iodide (PMAI) and [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) are introduced into the precursor. Compared with the precursor with no additives or a single additive (PMAI or 2PACz), the use of dual additives more effectively cleaves edge-shared Pb-I octahedra to expedite the transformation from PbI₂ to PbI₃ ⁻ complexes as prenucleation clusters and produces much larger colloidal particles with accelerated nucleation. Concurrently, the crystallization in both spin-coating and annealing processes is significantly retarded due to the stronger interaction between perovskite and binary additives. Benefiting from such rapid nucleation and slow crystallization, high-quality perovskite layer with larger-sized crystals and fewer defects is formed, resulting in mitigated microstrain, enhanced charge extraction, and suppressed nonradiative recombination. Consequently, the device derived from the use of dual additives could achieve an impressive efficiency of 26.05% (certified 25.49%) and retained 90% of its initial efficiency after 1200 h of maximum power point tracking.

The lack of effective hole-transporting layers (HTLs) materials has become a significant bottleneck in the advancement of organic solar cells (OSCs). Particularly, to obtain sufficient doping property, most HTLs have to be used by mixing with substantial amounts of acid, which causes serious corrosion problems. Herein, by optimizing molecular conformation, we designed and synthesized two conjugated polyelectrolytes (CPEs) that exhibited superior hole-transporting capability without involving the addition of acid. The PEP-BT features a planar conjugated backbone constructed from non-fused segment, which leads to an enhanced doping effect, highly ordered molecular arrangement and reduced π-π stacking from 4.13 Å to 3.85 Å. These contributed to the outstanding hole collection performance of polyoxometalate-doped PEP-BT:POM while maintaining a neutral pH. The binary OSC showed a photovoltaic efficiency of 19.16%, demonstrating an unprecedented example that a CPE-based HTL can provide a photovoltaic efficiency exceeding 19% in OSCs. Moreover, as an another evidence of high-performance HTL, the organic light-emitting diode by incorporating PEP-BT:POM exhibited a superior luminous efficiency to the reference device, along with a decrease of 0.4 V for turn-on voltage. The results from this work demonstrates the great potential of CPEs as high-performance HTL materials.

The molecular planarity, crystallization behavior and stacking orientation, as well as film morphology of hole transport materials (HTMs) can be regulated by constructing different noncovalent conformational locks. The use of TzTzTPA-NH, with its intramolecular hydrogen bonds, as a dopant-free HTM affords excellent PCEs of 24.2% for single-junction perovskite solar cells (PSCs) and 25.4% for perovskite/organic tandem solar cells (TSCs).

Abstract

Organic semiconductors with intramolecular noncovalent interactions are promising hole transport materials (HTMs) for efficient and stable perovskite solar cells (PSCs), but the effects of different types of noncovalent bonds on the properties of HTMs are rarely reported. Here, three thiazolo[5,4-d]thiazole (TzTz)-based HTMs with different side chains were developed. Compared with alkyl side chains, functional side chains can improve the crystallinity and charge transport ability of HTMs by forming intramolecular noncovalent interactions. However, the steric hindrance of S···O in TzTzTPA-SO distorted the molecular skeleton, leading to edge-on stacking and local aggregation of film. Fortunately, TzTzTPA-NH with intramolecular hydrogen bond showed high planarity, proper crystallinity, and preferred stacking orientation. Consequently, a remarkable power conversion efficiency (PCE) of 24.2% with a nice long-term stability was achieved by dopant-free TzTzTPA-NH-based PSCs, which is superior to the doped Spiro-OMeTAD-based PSCs. In addition, TzTzTPA-NH is well used as HTM in wide-bandgap PSCs and perovskite/organic tandem solar cells (TSCs). Encouragingly, the TSCs based on TzTzTPA-NH achieved an excellent PCE of 25.4%, which is the highest PCE of n-i-p perovskite/organic TSCs. This work clearly illustrates the effect of intramolecular noncovalent interactions on the properties of HTMs, and provides guidance for designing high-performance dopant-free HTMs in PSCs.

A simple and effective approach to grain boundary growth optimization: Incorporating a KTFB polyfluorinated additive into the antisolvent to regulate grain boundary growth, eliminate excessive halide lead and defects, and form wide-bandgap (1.78 eV) perovskite films with grain size over 2 µm, enabling the realization of efficient two-terminal and four-terminal all-perovskite tandem devices.

Abstract

Tandem perovskite solar cells represent a significant avenue for the future development of perovskite photovoltaics. Despite their promise, wide-bandgap perovskites, essential for constructing efficient tandem structures, have encountered formidable challenges. Notably, the high bromine content (>40%) in these 1.78 eV bandgap perovskites triggers rapid crystallization, complicating the control of grain boundary growth and leading to films with smaller grain sizes and higher defect density than those with narrower bandgaps. To address this, potassium tetrakis(pentafluorophenyl)borate molecules are incorporated into the antisolvent, employing a crystallographic orientation-tailored strategy to optimize grain boundary growth, thereby achieving wide-bandgap perovskite films with grains exceeding 2 µm and effectively eliminating surplus lead halide and defects at the grain boundaries. As a result, remarkable efficiency is achieved in single-junction wide-bandgap perovskite devices, with a power conversion efficiency (PCE) of 20.7%, and in all-perovskite tandem devices, with a two-terminal PCE of 28.3% and a four-terminal PCE of 29.1%, which all rank among the highest reported values in the literature. Moreover, the stability of these devices has been markedly improved. These findings offer a novel perspective for driving further advancements in the perovskite solar cell domain.

An n-doped ETL, that is c-NDI-Br:PEI with high electrical conductivity, strong work function modification ability and good solvent resistance, is developed via a simple in situ cross-linking reaction. The tandem OPVs, with the all-solution-processed organic/organic ICL (m-PEDOT:PSS/c-NDI-Br:PEI) achieve 20.06% efficiency under solar radiation, and 38.50% efficiency under 808 nm laser. This study provides a new method to design high-perfoimance ICLs.

Abstract

With merits of good solution processability, intrinsic flexibility, etc, organic/organic interconnecting layers (ICLs) are highly desirable for tandem organic photovoltaics (OPVs). Herein, an n-doped cross-linked organic electron transport layer (ETL), named c-NDI-Br:PEI is developed, via a simple in situ quaternization reaction between bromopentyl-substituted naphthalene diimide derivative (NDI-Br) and polyethylenimine (PEI). Due to strong self-doping, c-NDI-Br:PEI films exhibit a high electrical conductivity (0.06 S cm⁻¹), which is important for efficient hole and electron reombination in ICL of tandem OPVs. In addition, the cross-linked ETLs show strong work function modulation ability, and good solvent-resistance. The above features enable c-NDI-Br:PEI to function as an efficient ETL not only for single-junction OPVs, but also for tandem devices without any metal layer in ICL. Under solar radiation, the single-junction device with c-NDI-Br:PEI as ETL achieves a power conversion efficiency (PCE) of 18.18%, surpassing the ZnO-based device (17.09%). The homo- and hetero-tandem devices with m-PEDOT:PSS:c-NDI-Br:PEI as ICL exhibit remarkable PCEs of 19.06% and 20.06%, respectively. Under 808 nm laser radiation with a photon flux of 57 mW cm⁻², the homo-tandem device presents a superior PCE of 38.5%. This study provides a new ETL for constructing all-solution-processed organic/organic ICL, which can be integrated in flexible and wearable devices.

The rational toughening of photosensitive films is crucial for the development of flexible organic solar cells. Herein, a fine-grain strengthening strategy is demonstrated for mitigating the excessive aggregation or crystallization in small-molecule acceptor films, thereby suppressing the non-ideal thermodynamic behavior and residual-enriched state. Thus, these provide the potential for the synergistic enhancement of efficiency, mechanical and environmental stability in organic photovoltaics.

Abstract

The rational toughening of photosensitive films is crucial for the development of robust and flexible organic solar cells (F-OSCs), which are always influenced by mechanical strain and thermodynamic relaxation within the films. Nevertheless, the potential determinants of these properties and quantitative metrics modulating the overall performance of flexible devices have not been thoroughly defined. Herein, a fine-grain strengthening strategy is demonstrated for mitigating the excessive aggregation or crystallization in small-molecule acceptor films, the secondary thermal relaxation of side chains in polyethylene oxide (PEO) local motion restricts the free fluctuation volume through hydrogen-bonding interactions, thereby suppressing the non-ideal thermodynamic behavior and residual-enriched state. These contribute to an increase in yield strength and a reduction in microcracks while enhancing the fracture energy at the donor/acceptor interface. Finally, the optimal F-OSCs demonstrate champion PCEs of 19.12% (0.04 cm²) and 16.92% (1.00 cm²), and maintain 80% of their initial efficiency after heating at 85 °C for 2600 h. Besides, the flexibility and mechanical robustness of devices are also optimized, the elastic modulus and stiffness are decreased by 50.68% and 5.71%. This work provides interesting references for the synergistic enhancement of efficiency, mechanical and environmental stability in flexible organic photovoltaics.

Nature Communications, Published online: 20 March 2025; doi:10.1038/s41467-025-58047-3

Nature Photonics, Published online: 21 March 2025; doi:10.1038/s41566-025-01644-x

Nature Energy, Published online: 13 March 2025; doi:10.1038/s41560-025-01742-8

Nature Energy, Published online: 18 March 2025; doi:10.1038/s41560-025-01736-6

Publication date: 19 March 2025

Source: Joule, Volume 9, Issue 3

Author(s): Shuangyan Hu, Wanli Li, Shunchang Liu, Zhiwen Zhou, Yaokang Zhang, Ziqing Luo, Huanyu Jin, Qun Jin, Yi Hou, Xuechang Zhou, Zaiwei Wang

A crystallization-kinetics modulation strategy has been developed to synergistically regulate the hierarchical morphology in a typical bulk heterojunction blend, via introducing a polymer donor and a small molecule acceptor as the nucleation agent and plasticizer, respectively. Consequently, a decent device efficiency of 20.1% is obtained.

Abstract

Obtaining controllable active layer morphology plays a significant role in boosting the device performance of organic solar cells (OSCs). Herein, a quaternary strategy, which incorporates polymer donor D18-Cl and small molecule acceptor AITC into the host D18:N3, is employed to precisely modulate crystallization kinetics for favorable morphology evolution within the active layer. In situ spectroscopic measurements during film-formation demonstrate that while D18-Cl works as a nucleator to promote aggregation of D18 and foster donor/acceptor intermixing, AITC has exactly the opposite impact on aggregation of N3 and intermixing kinetics of donor and acceptor, working as a plasticizer. The mutually compensational effect of the dual-guests, as a result, enables synergistic control over fibrillar networks, multi-length scale morphology, and vertical phase distribution, leading to optimized 3D morphology for greatly enhanced exciton dissociation and charge transfer, suppressed charge recombination, and reduced energy loss. Consequently, the quaternary OSCs based on D18:D18-Cl:N3:AITC achieved an excellent power conversion efficiency of 20.1%, which represents one of the highest efficiencies for single-junction OSCs. This work presents an effective strategy to precisely regulate crystallization kinetics toward advanced morphology control for high-performance OSCs.

In this, we propose a dynamic passivation strategy that employs a pre-passivator, formamidinesulfinic acid (FSA), to synchronize the release of additives with the generation of defect sites, thereby enabling in situ and real-time defect passivation. A champion perovskite solar cell achieves an impressive solar conversion efficiency of 25.33 %, accompanied with an exceptional stability.

Abstract

Modifying perovskites with functional additives has proven effective in refining the crystallization process and passivating the defects of perovskite films, thereby ensuring high photovoltaic efficiencies. However, conventional methods that involve pre-mixing additives into the precursor solution often face challenges due to discrepancies in the spatial distribution of slow-diffused additives relative to dynamically formed defect sites, resulting in limited passivation effectiveness. To address this issue, this study innovatively proposes a dynamic passivation strategy that utilizes a pre-passivator to gradually release active additives during the thermal crystallization process of perovskite films. By leveraging the principle of energy minimization, these timely-released additives can interact precisely and selectively with the high-energy defect sites generated during crystallization, thus facilitating efficient additive utilization and in situ real-time defect passivation. Through analysis of crystallization kinetics and carrier dynamic, it is demonstrated that this dynamic passivation approach significantly improves film quality and prolongs carrier lifetime, outperforming traditional pre-mixing tactics. Consequently, the final perovskite solar cell achieves an impressive solar conversion efficiency of 25.33 %, along with exceptional stability. This work provides strong support of tailored additive strategies aimed at further enhancing the efficiency of perovskite solar cells and their subsequent commercial applications.

We systematically explored the impact of binding strength and acidity of a library of molecules on the wide-bandgap perovskite surface to fabricate efficient and hysteresis-free perovskite/silicon tandem solar cells. Ultimately, (2-phenylethyl)phosphonic acid (PEPA) has the optimal passivation effect and enhances the charge extraction capability, achieving a remarkable power conversion efficiency of 32.13% (certified 31.72%) for perovskite/silicon tandem solar cells.

Abstract

Passivating defects at the wide-bandgap perovskite/C₆₀ interface without impeding interfacial charge transport can effectively enhance the efficiency of perovskite/silicon tandem solar cells (TSCs). Herein, we study the impact of benzene-derivative ligands with elaborately modulated binding strength and acidity on wide-bandgap perovskites for high-performance perovskite/silicon TSCs. Specifically, the acidity/alkalinity and binding strength are preliminarily tuned using different functional groups of -PO₃H₂, -COOH, and -NH₂, and further finely adjusted by altering the chain lengths between the benzene ring and the functional groups. The results show that strong binding is indispensable for effectively suppressing voltage loss. However, the commonly used benzylphosphonic acid (BPPA) for firm surface binding exhibits too strong acidity that can etch the perovskite surface, resulting in halide-vacancy defects and pronounced hysteresis. Increasing the side chain length of BPPA to (2-phenylethyl)phosphonic acid not only enables a suitable acid dissociation constant (pKa) to avoid acid-induced etching but also achieves robust anchoring to the perovskite surface with a parallel adsorption orientation, which reduces the charge transport barrier at the interface. These properties enable strong-adsorption surface termination (SAST) of the perovskite surface while preventing acid-induced etching. As a result, the SAST strategy achieves a remarkable efficiency of 32.13% (certified 31.72%) for hysteresis-free perovskite/silicon TSCs.

A polymer random-walk model is introduced to evaluate the cumulative dipole moment of polymer hole transport material, revealing that PTAC-DEG with shorter glycol side chains achieves a larger polymer dipole moment, enabling 25.8% efficiency and exceptional stability in inverted perovskite solar cells.

Abstract

The dipole moment of hole transport materials (HTMs) plays a vital role in improving the photovoltaic performance of inverted perovskite solar cells (PSCs). However, manipulating dipole moments of polymer HTMs, which have great operational and bias stability, remains challenging. In this work, polymer HTMs with varying lengths of glycol side chains, PTAC-DEG and PTAC-TEG, are developed. Using a random-walk model extended from polymer chain conformation, PTAC-DEG (with shorter glycol side chains) achieves a higher cumulative dipole moment than PTAC-TEG. This finding can be attributed to the fact that smaller dipole angles in PTAC-DEG enable better dipole alignment, while larger dipole angles in PTAC-TEG promote dipole aggregation that reduces the integrated dipole moment. Perovskite films on PTAC-DEG show enhanced crystallinity and lower trap density due to improved interfacial charge transport, a stronger built-in electric field, and a better affinity due to the higher polymer dipole. PTAC-DEG devices achieve a PCE of 25.8%, which is among the highest for PSCs based on polymer HTMs, and the devices exhibit outstanding stability under ISOS-L-3 (t₉₅ = 1300 h) and ISOS-D-3 (t₉₅ = 1200 h) conditions. This study highlights dipole moment modulation as a promising strategy for designing efficient, stable polymer HTMs for perovskite solar cells.

The SnO₂/ZnSb₂O_6-x double electron transport layer promotes a highly ordered arrangement of the perovskite lattice, improving film quality and suppressing non-radiative recombination, which in turn enhances carrier transport and overall device performance. Under photothermal conditions, ZnSb₂O_6-x effectively captures oxygen atoms at the SnO₂ interface, preventing ion migration and thereby enhancing the photothermal operational stability of PSCs.

Abstract

Stability testing protocols from the International Summit on Organic and Hybrid Solar Cell Stability (ISOS) are essential for standardizing studies on the photothermally operational stability of perovskite solar cells (PSCs). Under photothermal conditions, the migration of oxygen from SnO₂ layer induces cationic dehydrogenation at the A-site of the perovskite, accelerating degradation to PbI₂. This leads to the formation of photoinduced I₂ and Pb⁰ defects, significantly compromising long-term stability. In this study, ordonezite (ZnSb₂O_6-x) as a multifunctional electron transport layer (ETL) that captures migrating oxygen atoms at the SnO₂/perovskite interface is introduced, effectively preventing degradation of the buried interface. Additionally, the lattice match between ZnSb₂O_6-x and perovskite facilitates well-ordered perovskite film growth. As a result, PSCs featuring ZnSb₂O_6-x ETLs achieved a high power conversion efficiency of 25.02% and retained 90.62% of their initial performance after 1000 h under the ISOS-D-2 protocol. Furthermore, devices demonstrated remarkable thermal stability, maintaining 83.69% of their original performance after 800 h of maximum power point tracking at 85 °C, meeting the stringent ISOS-L-2 protocol requirements.

Fluorobenzene side chain engineered Y6 type derivatives are presented to enhance the molecular packing and the morphology in organic solar cells (OSCs), thus achieving a remarkable 19.06% power conversion efficiency with any additive and providing a scalable but simplified approach for high-performance OSCs in the future.

Abstract

Organic solar cells (OSCs) have become a promising photovoltaic technology, achieving high efficiencies over 20%. However, simplifying processing techniques to maintain high performance remains a significant challenge. This work reports a series of Y6-derived non-fullerene acceptors (NFAs), namely BTP-R1F, BTP-R2F, BTP-R3F, and BTP-R5F, featuring fluorinated phenoxyoctyl side chains with varying numbers of fluorine atoms. Systematic fluorination has minimal impact on optical absorption and energy levels but significantly influences molecular packing and morphology. BTP-R1F, BTP-R2F, and BTP-R5F exhibit compact honeycomb-like stacking patterns with enhanced π–π interactions, while BTP-R3F displays a looser S-shaped stacking due to severe side chain folding, thus hindering charge transport. Additive-free OSCs processed with toluene demonstrate that D18/BTP-R5F has formed a well-defined fiber-like interpenetrating network, achieving a remarkable power conversion efficiency (PCE) of 19.06%. This study highlights the potential of fluorobenzene side chain engineering to enhance molecular stacking and morphology without any additive, offering a pathway toward scalable and high-performance OSCs with simplified processing conditions. The findings provide valuable insights for designing next-generation NFAs for efficient and reproducible OSCs.

Defect passivation is widely acknowledged as a crucial strategy for enhancing the efficiency and stability of perovskite solar cells (PSCs). However, it remains a formidable challenge to effectively address multiple defects simultaneously on both the top and bottom surfaces of perovskite films, as well as within the bulk, through a facile method. To tackle this dilemma, we have devised a triple passivation strategy, aiming to achieve a holistic passivation of defects at the aforementioned locations using a singular passivator. Specifically, a multifunctional molecule, tris(2,2,2-trifluoroethyl) phosphate (TTFP), is meticulously engineered as an additive in the antisolvent. This approach capitalizes a top-down gradient distribution of TTFP along the perovskite film, thereby enabling to mitigate the interfacial and bulk defects. Meanwhile, the unique molecular structure of TTFP facilitates simultaneous interactions with both cationic and anionic defects. Additionally, TTFP exerts a pronounced influence on the crystallization kinetics, thereby promoting the formation of highly crystalline perovskite films with substantially enlarged grain sizes. Consequently, the TTFP-based devices exhibit a champion power conversion efficiency (PCE) of 25.69%, accompanied by a notable improvement in stability. This work represents the successful implementation of comprehensive defect passivation, marking a significant instance in the advancement of efficient and stable PSC technology.

Publication date: 16 April 2025

Source: Joule, Volume 9, Issue 4

Author(s): Jin Hyuck Heo, Seok Young Hong, Jin Kyoung Park, Hyong Joon Lee, Fei Zhang, Sang Hyuk Im

The random terpolymer PM7-TTz50 balances structural disorder and robust molecular packing, enabling finely tuned nanoscale morphologies for high-performance organic solar cells (OSCs) using sustainable methods. It delivers PCEs exceeding 19% in OSC devices and over 16% in additive-free OSCs. Moreover, PM7-TTz50 is broadly compatible with non-fullerene acceptors, enhancing efficiency and reproducibility compared to its parent polymer.

Abstract

Achieving commercial viability for organic solar cells (OSCs) requires non-toxic, non-halogenated solvent processing. However, poor solubility and suboptimal morphology of commonly used active layer materials have been limiting their non-halogenated solvent applications for high-performance OSCs. This study introduces a novel random terpolymer, PM7-TTz50, designed to overcome these challenges. By incorporating 50 mol% of a co-planar thiophene-thiazolothiazole (TTz) unit into the PM7 backbones, the resulting terpolymer achieves enhanced solubility in eco-friendly solvents. Furthermore, PM7-TTz50's strong aggregation tendency, coupled with high-boiling-point solvent processing—which prolongs aggregate/crystal growth—enhances molecular stacking and ordering. This approach supports efficient charge transport and minimizes non-radiative recombination, yielding power conversion efficiencies (PCEs) exceeding 19% and over 16% w/o solvent additives. Additionally, PM7-TTz50 demonstrates broad compatibility with various non-fullerene acceptors (NFAs), leading to enhanced material uniformity and reproducibility in device fabrication.

Carbon nanohorns (CNH) to enhance the doping level of Spiro-OMeTAD is reported. The unique asymmetry and polar structure of CNH not only enable effective charge transfer between CNH and Spiro-OMeTAD, also exhibit confinement effect to trap Li⁺ ions and O₂, promoting the consecutive chemical doping processes.

Abstract

The doping level of 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD), a commonly used hole transport layer in perovskite solar cells (PSCs), is crucial for its electrical conductivity and the final photovoltaic performance. The routine oxygen-oxidation doping strategy falls short in achieving high-level doping due to the sparsity and random distribution of dopants within the solid Spiro-OMeTAD film. Here the use of carbon nanohorns (CNH) as a promoter to significantly enhance the doping level of Spiro-OMeTAD is reported. The unique asymmetry and polar structure of CNH not only enable effective charge transfer between CNH and Spiro-OMeTAD, also exhibit confinement effect to trap Li⁺ ions and O₂, promoting the consecutive chemical doping processes. Corresponding carbon-based PSCs achieved a power conversion efficiency of 23.24% (22.51% certified), and demonstrated exceptional long-term durability, retaining 95.3% of the initial PCE (power conversion efficiency) after 1500 h of tracking at maximum power point (MPP) under 100 mW cm⁻² illumination.

Organic photovoltaics are a promising solar cell technology, but even the record devices with 20% efficiency have significant fill factor losses due to low active layer conductivities. In this Perspective, the authors describe the origin of these losses, discuss experimental methods for their quantification, and explain how to minimize them to optimize charge collection in organic solar cells.

Abstract

Organic photovoltaics (OPV) are a promising solar cell technology well-suited to mass production using roll-to-roll processes. The efficiency of lab-scale solar cells has exceeded 20% and considerable attention is currently being given to understanding and minimizing the remaining loss mechanisms preventing higher efficiencies. While recent efficiency improvements are partly owed to reducing non-radiative recombination losses at open circuit, the low fill factor (FF) due to a significant transport resistance is becoming the Achilles heel of OPV. The term transport resistance refers to a voltage and light intensity-dependent charge collection loss in low-mobility materials. In this perspective, it is demonstrated that even the highest efficiency organic solar cells (OSCs) reported to-date have significant performance losses that can be attributed to transport resistance and that lead to high FF losses. A closer look at the transport resistance and the material properties influencing it is provided. How to experimentally characterize and quantify the transport resistance is described by providing easy to follow instructions. Furthermore, the causes and theory behind transport resistance are detailed. In particular, the relevant figures of merit (FoMs) and different viewpoints on the transport resistance are integrated. Finally, we outline strategies that can be followed to minimize these charge collection losses in future solar cells.

Novel polyimide-ionene hybrids that exhibit desirable alcohol processability, conductivity, transparency, and metal/semiconductor interface modification ability are furnished by melding pyromellitic diimides into ionene backbones. These merits render them universal cathode interlayer materials with outstanding thickness tolerance, leading to not only highly efficient and stable opaque devices but also high-performance semitransparent devices even when pairing with low-cost Cu electrodes.

Abstract

The contradiction between high transmittance and favorable conductivity poses a great challenge in developing effective cathode interlayer (CIL) materials with sufficient thickness tolerance, which hinders the further advancement of organic solar cells (OSCs). Herein, a completely new class of alcohol processable polyimide-ionene hybrids (PIIHs) is proposed by melding pyromellitic diimide (PMD) subunits into imidazolium-based ionenes backbone covalently. These PIIHs, named PMD-DI and PMD-PD, boast high transparency, suitable energy levels, and decent conductivity. A higher PMD content endows PMD-PD with improved work function tunability, electrical properties, and crystallinity, enabling PMD-PD as CIL material with excellent thickness-insensitive characteristics, while simultaneously improving device stability significantly. Furthermore, PMD-PD also exhibits good compatibility with various electrodes and active layers, offering solar cell efficiencies of up to 19.91% and 19.29% with Ag and Cu cathodes, respectively. More importantly, the application of PMD-PD can improve the performance of semi-transparent OSCs without losing transmittance, thereby drastically enhancing the light utilization efficiency to 4.04% with an ultrathin, low-cost Cu cathode, that competes with leading optical modulation-free semitransparent OSCs with expensive Ag cathodes. This work opens a pathway to realize transparent and conductive interlayers by strategic molecular design, leading to highly efficient, stable, and cost-effective OSCs suitable for diverse applications.

Nature Materials, Published online: 04 March 2025; doi:10.1038/s41563-025-02177-y

Nature Materials, Published online: 04 March 2025; doi:10.1038/s41563-025-02163-4